This invention relates to a directional antenna steerable in two directions.
An antenna is a means for radiating or receiving radio waves. It provides a transition means between radio waves traveling in free space and radio waves traveling in a transmission line. An antenna is characterized in part by its radiation pattern which can be depicted by a graphical representation of the directions in which energy radiates from the antenna as a function of space coordinates about the antenna. Directional antennas, for example, prefer to radiate more in one region of space than in another. The pattern will reflect this by containing a main beam in the pattern pointing in the preferred direction of radiation. It should be noted that antenna sensitivity to incoming RF energy transmitted to it from different directions is characterized by a receive pattern which is identical to the radiation pattern of the antenna. In other words the antenna will be more sensitive to reception of energy being transmitted to it along its main beam than energy being transmitted along other directions. As used herein the term antenna pattern is used for an antenna receiving or transmitting RF energy. When an antenna is transmitting, an RF signal source is connected by a transmission line to a feed portion of the antenna. When receiving RF energy, a receiver is connected to the feed portion of the antenna.
It is sometimes desirable to rotate a directional antenna about one or more axes to point the main beam of the antenna in a different direction. This process may include rotating the antenna continuously through a range of angles about the axes of rotation (called sweeping or steering the antenna). When the antenna is rotated, the associated transmitter or receiver usually remains stationary. Hence, an RF rotary coupler (called a rotary joint) is provided between the rotating antenna and the stationary receiver or transmitter. However, rotary joints present problems since RF energy must be transmitted therethrough during rotation without attenuation and unwanted RF reflections. As the rotary joint is used more and more, parts within the joint, which move relative to one another to accommodate rotation, and which make continual electrical contact to accommodate RF transmission, begin to wear causing intermittent noise spikes to the RF transmission and eventually total failure. Some examples of RF rotary couplers or joints are given in U. S. Pat. Nos. 2,434,925; 2,473,443; 2,523,320; 2,784,383; 2,812,503; 2,830,276; 3,011,137; 3,042,886; and 4,020,431.Applicant's invention provides an improved steerable antenna having an alternative to conventional RF rotary joints.